The present invention relates to an arrangement for analyzing the nonlinear properties of a communication channel.
It is known in the conventional art to generate test signals (probing signals) for the purpose of determining the nonlinear properties of communication channels or their analog preliminary stages, the test signal transmitted via the communication channel being evaluated in each case. In the past, sinusoidal signals, in combination with bandpass filters have been used to determine the nonlinear properties of communication channels. In this case, however, reliable determination of the nonlinear response of the communication channel is possible only when a multiplicity of different measurements is carried out in conjunction with different frequencies of the test signal. This mode of procedure is, therefore, relatively complicated, as well as costly and time consuming.
In order to test the nonlinear properties of the communication channel, the test signal should have a relatively large number of peaks in a suitable amplitude range. It has therefore been proposed in U.S. Pat. No. 5,515,398 to measure the power of the signal peaks and compare it with the root mean square average value (RMS) of the power of the test signal. A test signal for analyzing the nonlinear properties of the corresponding communication channel is optimized whenever it corresponds to a minimum ratio of the signal peak power to the RMS value. This ratio value is also denoted as the PAR (Peak-to-RMS average ratio) value. In the signal range of interest, a test signal with a minimum PAR value produces the strongest interference generated by nonlinearities in the communication channel, with the result that this interference can be most effectively detected and evaluated.
As described in U.S. Pat. No. 5,515,398, the test signal is composed of a multiplicity of individual tones whose tone frequencies lie in each case inside what is termed the Nyquist interval [0 . . . Fs/2], where Fs corresponds to the sampling frequency of the test signal. Some of these discrete tone frequency values are not used for the test signal, however. In the case of those tone frequencies not used for the test signal, intermodulation distortions and harmonic distortions are produced by the nonlinear properties of the communication channel or its analog preliminary stage. The strength of these nonlinear distortions in relation to the signal amplitude in the case of those tone frequency values that were not omitted and, thus, used for the test signal, can be used as a measure to evaluate the nonlinearity of the communication channel.
In order to determine the previously named nonlinear interference, it is customary to use an evaluation or detector circuit that comprises a bandpass filter of high selectivity, such as complex FIR filter or IIR filter of higher order, for example. Thus, the design of the detector circuit is relatively complicated.
The present disclosure proposes an arrangement for analyzing the nonlinear properties of a communication channel that affords reduction of the outlay on circuitry over the previously mentioned evaluation or detector circuits.
According to an aspect of the disclosed arrangement for analyzing nonlinear properties of a communication channel, a test signal generator device is encoded and configured to generate a test signal comprised of a plurality of different tones and send the test signal via the communication channel. The tone frequencies of each of the plurality of different tones of the test signal are set to values from the set of i*Fn/n where i=1 . . . n/2xe2x88x921 and Fs denotes the sampling frequency and wherein at least one of the tone frequency values from the set of i*Fs/n is not used for a tone of the test signal. An evaluation device is also included and configured to receive and evaluate the test signal transmitted via the communication channel. The evaluation device includes a first detector unit configured to detect the interference produced by the communication channel for tone frequency values that are not used for the test signal. Also included in the evaluation device is a second detector unit configured to detect test signal amplitudes occurring for each of the tone frequency values used. Further, a comparator unit is included in the evaluation device to relate respective output signals of the first and second detector units to one another in order to determine the nonlinear properties of the communication channel. Moreover, the first detector unit has one or more frequencies spectrum shifting units configured to shift the frequency spectrum of the test signal transmitted via the communication channel by the individual tone frequency values not used for the test signal. The first detector unit further includes one or more low-pass filters connected downstream of corresponding frequency spectrum shifting units in order to subject each frequency-shifted test signal to low-pass filtering. The first detector unit outputs a sum of the individual frequency-shifted and low-pass filtered test signals as an output signal to the comparator unit.